(1) Field of the Invention
The present invention relates to an optical module, and more particularly to a multiwavelength optical transmitter module that multiplexes and transmits lights of plural wavelengths, and a multiwavelength optical receiver module that demultiplexes and receives a light obtained by multiplexing the plural wavelengths.
(2) Description of the Related Art
In the recent information communication field, a communication traffic that reciprocates large volumes of data at a high speed by the aid of lights is rapidly improved. In particular, broadbandization of an access line associated with an explosive spread of the Internet is accelerated, and the remarkable rising of an FTTH (fiber to the home) service on the market has been found. Among the optical transmission systems of the FTTH, a PON (passive optical network) system in which plural subscribers share one optical fiber is now growing in demand. In that system, data that has been transmitted from a central office through one optical fiber is split into 32 optical fibers from 16 optical fibers by a splitter, and distributed to respective subscriber homes. This makes it possible to remarkably reduce the optical fiber laying costs. Also, an ONU (optical network unit) is laid on each subscriber side as a terminal device, and a downlink signal (wavelength 1.5 μm) to each subscriber side from the central office and an uplink signal (wavelength 1.3 μm) to the central office from the subscriber side are subjected to wavelength division multiplexing (WDM) to transmit the uplink and downlink signals by the aid of the same optical fiber. Further, a three-wavelength bidirectional optical module is located within the ONU, which is basically made up of a light emitting element (LD: laser diode) for transmission of the uplink signal, a light receiving element (PD: photo detector) for reception of the downlink signal, and a WDM filter that separates the uplink and downlink signals from each other.
A conventional module system is shown in FIG. 1. FIG. 1 shows a basic configuration of a single-core bidirectional (BIDI: bidirectional) module where the respective optical parts of a light emitting element 11, light receiving elements 12a and 12b, and wavelength selection filters 31a, 31b are spatially arranged within a package 50. In this system, because the respective optical parts can be fabricated independently, the fabrication yield can be easily ensured. Also, an optical link is enabled by so-called active alignment that optically axially adjusts the optical elements 11, 12a, and 12b each packed into a CAN package having a lens 40 integrated therein to an optical fiber 60 while operating the optical elements 11, 12a, and 12b. This leads to the advantage that stable optical coupling efficiency is obtained. On the other hand, the number of parts is larger. In addition, because the wavelength selection filters 31a and 31b are mounted obliquely with respect to end surfaces of a package 50, there is a need to process the package 50 to be at a desired angle with respect to a Z-axis shown in FIG. 1. This causes a drawback that is disadvantageous for a reduction in the size and costs.
In order to achieve both of the extendability of the wavelength and the reduction in the size and costs, there is a need to form the wavelength multiplexer and demultiplexer in a compact space. For provision of the compact wavelength multiplexer and demultiplexer, there is a method of mounting plural filter units on a common parallelogram prism or another optical block. For example, in a multiplexing device disclosed in JP-A-Sho-61-103110, a plurality of optical filters split lights different in wavelength, which have propagated through a common optical waveguide. The multiplexing device has a multiplexer and a demultiplexer in which wavelength selection filters and mirrors are fixed at given positions of a transparent substrate so as to transmit lights of predetermined wavelengths and reflect lights of other wavelengths, respectively. The wavelength selection filters and the mirrors are arranged in such a manner that an incoming light is input to the transparent substrate, and sequentially partially transmitted and partially reflected by the respective wavelength selection filters and the mirrors disposed on a surface of the substrate to form a zigzag optical path. A light of a specific wavelength is removed or added by each of the filters. However, this multiplexing device is configured so that the optical elements and the optical multiplexer and demultiplexer are coupled with each other through rod lenses or optical fibers, which makes a reduction in the size difficult and the number of parts large.
As described above, the related art is large in the number of parts, and large in the number of processes for mounting the optical parts including mounting of the optical elements. Also, for the purpose of mounting the wavelength selection filters obliquely with respect to the end surface of the package, there is a need to machine the package so as to be at the desired angle with respect to the Z-axis shown in FIG. 1. Therefore, because the processing is difficult and the costs are high, there is a limit to the reduction in the size and costs.